"Those who believe the power of God is unable to keep them saved, do not understand the true nature of salvation. "
That's not the point at all. I am convinced God could have kept Eve from sinning and that there would have never been a need for Christ to come for our Salvation. Did God do that? No, so supposing that because God is capable of a thing He will do that very thing is not relevant in the least. God gives us the free will to accept Salvation then to continue to believe and live as if we are believers. Those who don't are either once again in a fallen state or they're not, that's the issue.
Of course God has the power to save us and to keep us from falling but you can find plenty of folks in prison for murder who will tell you they were "born again" as a child and now don't believe in Christ because He let them fall. So, did God keep them from falling or did God allow them to be deceived into thinking they were saved when they were not? We have a role to play as well and the limits of our role is what is at issue when you talk about OSAS, not the limits of what God can do.
You fail to understand there is a vast difference between the person who has been born again, the person who has a divine nature, verses Adam & Eve, even in their pre-sin condition. Christ was the second Adam, and much more than Adam, who was not God in the flesh. Christ is infinitely greater than Adam. In our glorified state, we will be incapable of sinning, because we will be like Christ. Currently, we must deal with the unredeemed flesh. Once a person is saved, Paul speaks of his salvation as being a completed deal. Whom God justified, He sanctified and GLORIFIED. God justifies FOREVER, a man the very moment he is born again. In the eyes of God, it’s already done! Whom He justified, He glorified. Paul speaks as if it’s ALREADY DONE — PAST TENSE. Adam had a choice to obey or disobey God, he disobeyed and died. The second Adam (Christ), via the cross offers man a choice. Be resurrected spiritually, or die lost and spiritually dead. Once the choice is truly made for redemption, there is no turning back. It’s similar to the angels who fell, they made their final decision, and redemption is not available for them. We were spiritually dead due to Adam’s sin, “God loved us even while we were yet sinners,” hence, he offered us redemption. Once we truly make the choice, it’s final, done, and cannot be reversed. It is God who initiates salvation. Glorification is guaranteed for those whom God justifies, and we’re fully justified upon repentance and acceptance of Christ. A decison for Christ is a one time, done deal that cannot be reversed.